This invention relates to the casting of large metal parts such as ingots.
When ingots are cast in a mold, the exposed top of the ingot cools much more rapidly than its main body, and as a consequence, the upper part of the metal mass adjacent the open top of the mold contracts more rapidly than the main body of the ingot. This results in a cavity or "pipe" which extends from the top surface of the ingot down into its interior. The shrinkage pipe forms early in the process of solidification. Because of oxidation and other chemical effects on the surface of the shrinkage pipe, the material surrounding the pipe cannot be rolled successfully because the pipe forms an unbonded crack within the rolled ingot.
The most commonly employed method of overcoming this problem involves cutting away the top portion of the ingot and recycling the material surrounding the shrinkage pipe. The loss of this material severely affects the metals industry. For example, in steelmaking the loss of ingot tops due to shrinkage pipe formation reduces manufacturing yields by 20 to 25 percent. While this portion of the metal may be recycled, it nevertheless represents a substantial cost in steel making since in order to produce an ingot for rolling of e.g., 100 tons, approximately 120 tons of steel must be cast.
To overcome this problem, some steel producers burn thermite or similar incendiary material directly on top of the ingot to prevent the rapid cooling which produces the shrinkage pipe. This approach is costly and only partially effective in reducing shrinkage. It is also characterized by a significant environmental problem because metallic particulates and gases are evolved as the incendiary material burns. These are difficult and costly to control.